The typical way that screenplays are put together is based on the Greek tragedy, three-act model. In Act One, the characters and the conflict that they are facing are introduced to the audience. The main action of the screenplay, however, takes place in Act Two. This is the longest and most intensive act of the screenplay.
The conflict rises to the forefront, and the character keeps fighting to overcome this conflict until a climax is reached. Once the climax is reached, Act Three ensues. This is where the conflict has been resolved, and a conclusion is reached.
While this is the main breakdown of a screenplay, you want to make an outline before you begin writing your script. The outline is basically just a skeleton of how your scenes are going to break down. While the outline helps you work your way through your premise, it is not definitive. You can deviate from your skeleton and probably will once you start writing. This is meant as only a guideline.